1. Field of the Invention
This invention relates generally to data transmissions utilizing a circuit switched network and more particularly to an interface system for preventing loss of a data session when a line is temporarily switched for an optional use.
2. Background Art
Among the many circuit switched network service features offered by telephone companies and some private exchanges which have been made available to a single line telephone subscriber have been the ability to switch between an existing call and a second incoming call which is "waiting" or placing an existing call on hold while making an outgoing call. A single line telephone subscriber was provided with a single telephone access number which supported a single voice grade circuit. The single line telephone instrument and/or equipment was connected to the telephone company central office and telephone company equipment over a local loop and supported analog transmissions on the local loop. In the "call waiting" situation, during the subscriber's initial conversation the telephone central office transmitted an audible "call waiting" signal to the subscriber. Upon receipt of such signal, the subscriber had the option of placing the existing call on hold and answering the incoming call. This was accomplished by toggling, i.e. pressing and releasing the hook switch in a predetermined sequence. The toggling thus constituted a communication with the central office instructing the telephone company equipment to alter the communications path. The central office then placed the existing call on hold and switched the incoming call to the subscriber's telephone line. Upon completion of the second call, the central office was again toggled to reinstate the initial call.
To utilize the call hold, conference call or three way calling optional service feature, the subscriber placed an existing call on hold, by toggling the central office, obtained a dial tone to initiate a second call and if the two calls were to be conferenced, again toggled the central office to effect conferencing. If the calls were not to be conferenced the second call was terminated and the original call reinstated by toggling.
Such telephone subscriber features, while convenient, presented problems when the initial call comprised a data session between a subscriber's personal computer terminal and a host computer. Often the incoming "call waiting" signal introduced data transmission errors at either the modem or the terminal. Not only did the "call waiting" signal interfere with the data transmission but, in addition, it was virtually impossible for the subscriber to access the incoming call waiting or even determine who the caller was without loss of the data session.
If the subscriber toggled the central office, not only did the toggle signal itself provide an error source, but, in addition, since neither the host nor the terminal was programmed to recognize that the central office placed their connection on hold status on their existing lines, carrier signals were lost and the communication was terminated. Such loss of communication resulted in unnecessary expense to the subscriber and generally required the subsequent reestablishment of another data session where at least a portion of the prior data transmission had to be duplicated. The same problems existed if a subscriber wished to initiate a call hold feature while on a data session.
When either feature was attempted to be utilized, the carrier signal between the terminal modem and the host modem was broken. Since the communications path through the telephone network was suspended, the carrier signal was lost causing the modems to disconnect the session.